Paired lines are a conventional means of carrying telecommunications transmissions. A paired line is made up of two balanced conductors individually insulated and twisted together. Paired lines are typically bunched together in a cable termed a paired cable which contains up to one hundred or more paired lines, wherein each paired line is capable of independently carrying telecommunications signals. Paired lines are generally effective telecommunication carriers, however, it is not unusual for noise to occur in paired lines which is extremely disruptive to the clarity of the transmitted signal.
Transmission quality of a telephone line is directly proportional to the balance of the pair of conductors with respect to earth ground, or central office frame ground. A perfectly balanced pair has equal impedance on each conductor (tip and ring) with respect to ground, over the range of frequencies of interest, e.g. for voice grade lines the frequency range of interest is approximately 300 Hz to 3000 Hz. Imbalance results in noise on the line. When noise is reported in a paired telecommunications line, correction of the condition requires confirming the presence of the noise in the line by measuring its level and then isolating and locating the noise source for purposes of eliminating it. There is a wide range of noise sources for which detection is desirable since virtually any condition which can cause an imbalance between two conductors of a paired line can result in noise. Among the causes are series resistance faults, shunt resistance faults, cross faults, shunt capacitance faults, unbalanced series inductance, and power influence. These faults may, for example, be caused by water in the cable sheath, improper cable splicing, or faulty equipment attached to the pair.
Series resistance faults occur when there is a poor connection at a splice or wire termination, often resulting from a corroded joint. Shunt resistive faults occur when another body grounds a paired line. Cross faults occur when there is communication between adjacent paired lines in a cable. Shunt capacitance faults occur when one conductor of a pair is slightly longer than the other conductor, and the longer conductor possesses a higher capacitance to ground than the shorter conductor. Unbalanced series inductance occurs when only one half of a load coil is connected to a paired line at some point along the length of the line. Power influence is induced voltage from an ac power source adjacent to the paired line. Unlike the above-recited causes of imbalance, power influence imbalance can occur even when the paired line is apparently free of faults and appears balanced in the absence of the power influence.
Power influence, which as noted above is induced voltage from line to ground, most commonly occurs when the paired line is near a power line. In the United States, the power line frequency is typically 60 Hz, but power influence can likewise result from other power line frequencies, including 50 Hz, as typically found in many other parts of the world. Power influence can create unique problems for noise detection when it occurs in conjunction with a fault. For example, a series resistance fault may only produce a high level of noise when accompanied by a high power influence. Therefore, a noise caused by the fault may be observed by a user at a time of high power demand on a nearby power line, but when a repairman is dispatched to the site, the power demand and correspondingly the power influence may have diminished so that the noise resulting from the fault alone is no longer detectable by conventional detection devices. Accordingly, such a fault is very difficult to locate and repair.
Another detection problem results from the fact that power influence signals often do not create large longitudinal current flow. Such flow is necessary to detect series resistance faults because longitudinal current flow through a series resistance fault produces a voltage imbalance in the paired line which can be measured metallically. However, because conventional passive detection devices lack the ability to independently generate longitudinal current flow, they accordingly may fail to detect such faults where power influence is relied upon to generate longitudinal current flow.
Various attempts have been made to detect imbalances in paired lines. For example, time domain reflectometer (TDR) tests have been utilized to detect imbalances on the conductors of the pair. In these traditional TDR tests, a metallic signal is applied to the line under test for a brief period of time and the technician monitors a receiver to determine if a reflection of the applied signal is received. A metallic signal is one which is applied the conductors. A reflection of the applied metallic signal provides an indication that a fault on the line was encountered. The reflected metallic signal is received and analyzed. By operating in this send and listen type format, the technician can determine the nature and location of the fault. Because the applied pulse signal in a traditional TDR is metallic, the technician cannot tell whether the return pulse represents an imbalance fault (from one conductor to ground) or a metallic fault (across the conductors of the pair). Imbalance faults produce a smaller reflection of metallic pulses than the reflections produced by a metallic fault of the same impedance, so imbalance faults greater than approximately 1000Ω can not be reliably located with a traditional or metallic TDR. Because imbalance faults act as noise injection points, DSL traffic is much more sensitive to imbalance faults than to metallic faults. For pairs used to provide DSL service, therefore, it is desirable to be able to detect imbalance faults.
Another test for detecting imbalances in paired lines is the “stress test”. The term stress test has become the accepted name for the test described in U.S. Pat. Nos. 5,157,336 and 5,302,905. The stress test provided a new way to test all cable pairs, working or dry, for proper balance. This test has become the telephone industry standard for determining the usability of a pair before placing it in service, and for isolating pair balance trouble to the source. A particular benefit of the stress test is in testing dry pairs before placing them into service as the test identifies “killer pairs” that tested good by previous methods yet tend to go bad within 48 hours after being placed in service.
Apparatus implementing the above stress test send out a simplex (both sides of the pair excited equally with respect to ground) “stress” tone through a balanced center tapped termination. Any imbalance on the pairs converted the simplex tone to metallic (across the conductors) which was amplified and filtered through a C Message filter. The filter output was converted to display either stressed noise or stressed balance, with stressed noise in dBrnC being the most popular.
The stress test simplex stress tone acted as an artificial “power influence” signal, permitting any pair's balance to be tested, even those pairs having too little power influence to allow a normal longitudinal balance reading. Longitudinal balance readings expressed the difference between passive power influence and noise metallic readings on the pair and thus did not place simplex excitation on the pair. The stress test internal termination to ground caused longitudinal current flow on the pair, revealing series resistance imbalances invisible to the longitudinal balance test. The pair can be tested from either end and does not require a termination in the central office.
The stress test concept for evaluating the voice transmission quality of a telephone line has been successfully applied in instruments such as Tempo's Sidekick® products. Tempo's Sidekick® testers have been used to stress voice frequency telephone lines by applying a 1 KHz simplex “stress” tone longitudinally, equal levels, same phase to tip and to ring, with reference to ground/cable shield. Any imbalance in the pair under test will result in a difference between signal amplitude on the tip and ring, and is measured across the pair (metallic). The larger the imbalance, the larger the 1 KHz metallic imbalance signal.
In addition, Tempo's Sidekick® testers provide for application of a high DC voltage along with the stress tone to determine the presence of moisture in the cable. If there is moisture present inside the cable sheath, the high voltage will create an ionization path and further degrade the balance of the pair under test. Normally, a good, balanced, non-wet pair will measure less than 30 dB stress, or less than 0.1% difference in levels across tip and ring as compared to the simplex stress tone.
A problem exists with the above stress test in that induction noise induced onto the tested pair in the voice band adds to the test signal converted from simplex to metallic by any imbalance on the pair causing high stressed noise and inferring poor pair balance when balance is not the source. In addition, high power influence can swamp out the applied simplex stress voltage causing erroneous high stressed noise readings. In areas with high power influence approaching or above the applied “stress” voltage, the stress test will erroneously read bad on good pairs. Thus, on noisy pairs you may not be measuring stressed noise, but induced noise converted from high power influence (50/60 Hz harmonics) on the pair due to the wideness of the C Message filter. This erroneous reading can cause technicians to try to improve pair balance rather than correcting high power influence, the true cause of the bad stress test indication. Therefore a “Voiceband Stress Test” is needed that can indicate the true stress balance of a pair in the voiceband with the presence of normal or high power influence.
Furthermore, the stress test as described above applies a simplex tone, in the voiceband typically near 1 kHz and indicates the balance of the pair at that frequency. Pairs that stress bad in the voiceband usually will not perform in the DSL band. A good stress test reading in the voice band however, does not necessarily indicate the pair will perform well in the DSL band. Minor capacitive or resistive imbalances that do not give a bad reading in the voiceband, can be service-affecting in the DSL bands.
A test is needed that performs similar to the voiceband stress test but is used for testing pairs in the DSL band and can therefore be used to isolate service-affecting DSL problems by technicians already familiar with using the stress test. Preferably, this test would quickly give a numeric readout allowing a confirmation that the pair is within parameters for, service.
A test is also needed which can detect higher impedance imbalance faults than a traditional or metallic TDR.
A test is needed which can be used on an in-service DSL circuit where a traditional TDR will not work.
For inactive pairs in a DSL service, a test is needed which can detect and locate imbalance faults in pairs for a wide range of frequencies, without requiring each frequency of interest to be separately examined.